1. Field of the Invention
This invention relates to improvements in intruder alarm systems.
Intruder alarm systems are liable to the occurrence of false alarm indications resulting from a variety of causes, and the elimination or reduction of such false alarms is a major requirement of such systems. False alarm signals result for the most part owing to the inability of the detecting devices used in the system to distinguish between an actual intrusion and spurious events producing similar responses by the detector. Such spurious responses may be due to the use of a detector responding to non-intrusive events occurring in the particular environment, by the maladjustment of the detector so that its detection threshold is too low, or by the incorrect selection of the region within which the detector is responsive. These false alarms are therefore originated either by the detector equipment itself, or by the incorrect installation or adjustment of the equipment.
2. Description of the Prior Art
Detection techniques used in intruder alarm systems include volumetric microwave or ultrasonic Doppler detection of movement within the protected area, passive infra-red sensors responsive to radiation emitted by an intruder, active infra-red detectors including an infra-red radiator and receiver, glass-break detectors, vibration detectors and other detector arrangements, all of which are capable of response to external technical influences which may result in an alarm signal being given.
For example, a vibration detector applied to a window may yield an alarm signal in response to a sonic boom caused by an airplane, to vibration caused by a heavy truck passing nearby, or to some similar non-intrusive event, or to vibration caused by an intruder's attempts to force the window open. Such a vibration detector does not include any intrinsic or external "intelligence" to enable it to distinguish between an actual intruder and vibration due to some other cause.
Generally, such a window vibration detector may have its sensitivity reduced so that its liability to giving false alarms is also reduced, but this will also reduce its response to vibration caused by an intruder and will thus make it easier for a skilled intruder to circumvent its operation.
Generally, the more sensitive is a particular detector device the more readily is it influenced to yield an alarm signal by disturbances other than those produced by an actual intruder and the less readily may it be circumvented by an intruder. It is therefore desirable in practice to be able to adjust the detector to a sensitivity which is as high as is compatible with absence of an excessive number of false alarms, and is therefore not readily circumvented. Appropriate adjustment of the detector is difficult because the requirements for high sensitivity and for a low rate of false alarms are diametrically opposed.
Consider, for example, an acoustic detector within an enclosed space, which is arranged to respond to specific audible frequencies of predetermined intensities and/or durations will not be able to discriminate against such audible signals originating in adjacent traffic movements, if it is set to a sensitivity high enough to respond to produce an alarm signal in response to noise made by an intruder in forcing an entrance or in moving within the space to be protected. What is required, therefore, is some form of discrimination capability or "intelligence", either built into the detector or otherwise incorporated in the alarm system, for example, within an alarm control panel, which will enable the system to discriminate between similar alarm signals due to different causes.
Taking as another example an ultrasonic Doppler intruder detector, such a device will respond not only to the presence of an intruder within the protected space but also to ultrasonic energy of frequency close to that used by the detector, such as may arise from ringing bells, or to the passage of a flying insect sufficiently close to the transmitter that substantial energy is reflected to the detector. A detector of this type will also be sensitive to changes in atmospheric conditions that affect the behaviour or propagation of ultrasonic energy within the protected space.
Ultrasonic Doppler detectors used at sensitivities such that they are capable of intruder detection at distances up to ten meters are found to be very easily influenced by the conditions mentioned above as giving rise to false alarms, and in practice such detectors are more usually adjusted to be capable of reliable intruder detection at distances not exceeding seven or more usually five meters, if a useful compromise between reliability of intruder detection and relative freedom from false alarms is required. It has been proposed to use very sophisticated signal processing circuits to operate on the detector signals in order to discriminate against responses not due to an intruder, but such arrangements render the detector costly, require skill and know-how for their correct installation and adjustment and give only slightly increased discrimination against false alarms. It is usual for such systems to include a central signal processing device into which the individual Doppler frequency-shift signal from a plurality of spaced detector units are fed.
Yet another example is a microwave Doppler intruder detector, which may have a high detection range owing to its electromagnetic propagation characteristics. Present microwave detectors are very liable to false alarms owing to their sensitivity to radio-frequency interference from transmitters, mobile radios, sparking of arc-welding equipment, lightning, flurorescent lamps in which the ionic discharge within the tube moves. False alarms may also arise from the penetration of the electromagnetic waves through the boundaries of the protected space, with the result that moving objects outside that space may give rise to alarm responses. For example, a motor-vehicle with a metal body, even though separated from the detector by substantial walls, may yet give rise to a greater response that that due to a human intruder within the protected space, owing to the differing efficiencies of metal and flesh as radar targets. Although this difficulty may be alleviated to some extent by appropriate adjustment of the sensitivity threshold of the detector, so that minimum responses are obtained from objects outside the protected space, yet such responses cannot be wholly avoided unless response to an intruder is limited to ranges of only a few meters. Once again, therefore, it is desirable to incorporate in the system a discriminator or "intelligence" capable of preventing responses to signals originating in causes other than an actual intruder.
From the examples given above it will be understood that no detector of a nature substantially more complex than a simple magnetic switch (and even such a switch may give rise to false alarms if incorrectly installed) in immune against false alarms.
Those engaged in the intruder alarm industry are currently engaged in applying much effort and skill to the development of increasingly complex and costly signal processing devices into individual detectors in order to reduce their false-alarm susceptibility. Such increased elaboration of the detector equipment leads, unfortunately, away from the manifest practical need for simple intruder detectors of high false-alarm rejection. Additionally, the more complex the apparatus the higher is its failure rate (or the lower is its reliability). Obviously, the less complex is the circuitry of a device the more reliable it will be.
In U.S. Pat. No. 3,727,216 and in U.K. Pat. No. 1,386,223 there are described intruder alarm systems employing both ultrasonic and electromagnetic Doppler detection systems and in which an AND circuit is used to correlate signals provided by the two systems so that simultaneous alarm responses by each of the two systems are required to yield an alarm output from the AND gate. This arrangement is clearly seen to be advantageous in that it greatly reduces the incidence of false alarms. It is also possible to employ other combinations of different detection systems, for example, a passive infra-red detector and a microwave detector, a passive infra-red detector and an ultrasonic detector, an interrupted beam infra-red detector and a microwave detector, an interrupted beam detector and an ultrasonic detector, since in each case the two systems of each combination yield false alarms in response to different effects.
These known systems suffer from both economic and operational disadvantages. The components of the combined systems are required to be so constructed as to provide similar logic signals and must therefore be individually designed for use in a combination with unavoidable increase in cost. It is also the case that an alarm will be given only when both systems are exactly simultaneously responsive to the presence of an intruder.